This invention relates to an electrophotographic copying apparatus (hereafter "photocopier") having a replaceable photoconductive sheet of finite length. The leading edge of the sheet is secured to a guide drum by means of a holding device. About the guide drum there are arranged, in close vicinity to one another, a charging station, an illuminating station, a developing station, a transfer station and a cleaning station where, respectively and in sequence, the photoconductive sheet is charged and photographically exposed to light, the image is developed by a magnetic toner, the image on the photoconductive sheet is transferred to a record medium and the photoconductive sheet is cleaned.
A large proportion of known and widely used photocopiers have a recording drum which carries on its cylindrical surface a layer of recording material, for example, a photoconductive layer. The drum rotates in a housing which also accommodates the various processing stations. Since, because of a deficiency inherent in the principle of photocopying processes, a complete transfer of the toner--which is applied for rendering visible the latent charge image--to the record medium (copy) is not feasible, the residual toner has to be removed from the drum by mechanical means, for example, with the aid of a brush. If such a toner-removing operation is not performed, a toner film builds up on the photoconductive layer, making necessary a frequent cleaning or premature replacemnt of the recording drum to ensure that the copy quality does not fall below a predetermined standard. It is another disadvantage of photoconductive layers provided firmly on the recording drum that a replacement of the entire drum is necessary as soon as the upper surface thereof is damaged.
A cleaning, an examination for damaged areas and a replacement of the recording drum can be performed only by qualified personnel; this circumstance increases the cost of an otherwise low-price copy obtained by an electrophotographic coyping process.
The above-discussed disadvantages are eliminated in known copiers by providing a photoconductive sheet maintained taut about the drum. Such an arrangement is disclosed, for example, in U.S. Pat. No. 3,600,086. The photoconductive sheet extends from a supply roll positioned inside the copier drum to the outside thereof, extends about the drum circumference, then enters the drum and is wound on a take-up roll. This arrangement provides for an automatic replacement of the photoconductive layer at the end of its useful life. This mechanically complex system is expensive and is used only in high-output automatic copiers which otherwise would have to be continuously attended by maintenance personnel.
Further, U.S. Pat. No. 3,883,242 discloses an electrophotographic copier which has a replaceable photoconductive sheet positioned on a drum-shaped guide component and provided at both ends with holder plates. After positioning the sheet about the cylindrical outer face of the drum, bore holes provided in the holer plates at one end of the sheet are brought into alignment with holder pins carried by the drum, while a spring at the other end of the image plate is hooked into a spring-supporting post also carried by the drum. Although in this manner, to be sure, the photoconductive sheet can be mounted securely on the drum, its involved replacement requires a certain skill and adroitness on behalf of the operating personnel.
Further, German Auslegeschrift (Published Accepted Patent Application) No. 2,406,162 discloses an electrophotographic copier including a photoconductive sheet glued together at its ends to constitute an endless band. This known arrangement, however, does not operate to cause the trailing end of the photoconductive sheet to be automatically connected with the leading end thereof.